The invention relates to a process for forming bioresorbable rods, pins, screws made of resorbable polymeric material such as polylactides and polyglycolides. More particularly, it relates to a continuous process for forming a rod, strip, tape or bar, which continuous rod, strip, tape or bar is cut into segments so that resorbable meshes, plates, pins, medullary rods, and screws maybe manufactured therefrom.
Various processes have been proposed to form bioresorbable or bioabsorbable thermoplastic rods, pins, screws and plates for use in orthopedics. These processes are either “batch” or continuous “processes”. A batch process is performed on preformed molded bodies whereas a continuous process makes strips, tape, bar or rods in a continuous manner. Continuous processes were first used to manufacture resorbable sutures. Such processes such as melt-processing or spinning are taught in U.S. Pat. Nos. 3,636,956 and 3,797,499. Melt-processing consists of melting in an extruder and extruding the material as a filament and then stretching the filament for orientation of the polymer chains. Melt-spinning is a term of art describing this process where absorbable sutures made of copolymers of L-lactide and/or glycolide are continuously extruded to form suture filaments which were then drawn at temperatures of between 50° C. and 140° C. at draw ratios up to 11 and then annealed.
U.S. Pat. Nos. 3,463,158 and 3,739,773 also relate to continuous processes for forming absorbable implants made of polyglycolic acid. As in U.S. Pat. Nos. 3,636,956 and 3,797,499, a melt spinning process is used to form filaments which are then drawn at about 55° C. to five times their original length. The diameter of these filaments may easily be increased to form self-supporting members or rods.
A continuous process for forming rods is also shown in European Patent Application EP 0 321 176 A2. This patent application relates to a continuous process and as shown in FIG. 4 thereof, a polylactic acid rod is continuously extruded and cooled and then drawn at a temperature above its glass transition temperature with tension being maintained on the polymer rod during cooling.
Various patents relate to a batch process in which a rod or bar is formed by molding, such as injection molding, and then later drawn to orient the polymer chains and strengthen the article. Such a process is disclosed in Ikada et al. U.S. Pat. No. 4,898,186, in which a poly-L-lactide shaped body is formed in a mold and is then axially drawn about 2 to 10 times at a temperature of 70° C. to about 120° C. Ikada et al. U.S. Pat. No. 5,227,412 also relates to a batch method in which molded articles are oriented after molding. A similar process is taught in Tormala et al. U.S. Pat. No. 4,968,317 in which poly-L-lactide rods are made by injection molding and then drawn at a draw ratio of 7 at temperatures up to 40° below the melting point of the polymer. All of the processes taught by the batch method patents suffer from low production rates, since each molded rod is individually heated and then drawn to orient the molecular chains.
It has been found that the continuous process of forming bioabsorbable rods, strips or tapes, while more productive than the molded process, was difficult to implement because the cross-section of the extrusion was difficult to control. This is the result of the extrusion process in which polymer granules are fed from a hopper into an extruder where they are heated and melted and then extruded usually by a screw. The flow rate from the hopper into the melting area of the extruder and then into the screw, which extrudes the material, is not constant for a variety of reasons, including that the weight of material in the hopper varies as material is withdrawn therefrom. This results in an extrusion having a non-uniform cross-section exiting the extruder and if a die is used to shape the extrusion, in turn, results in the extrusion leaving the die with a non-uniform cross-section.
As discussed below, because the extrusion is placed under a high tension during stretching or orientation, any non-uniformity in cross-section results in high stress areas within the extruded rod or strip which may result in the rod or strip breaking. Once the strip or rod extrusion breaks, the entire process must be stopped and many feet of expensive extrudate and time are wasted and these must be removed from the process line.
The process of the present invention solves this problem by placing a metering pump downstream of the extruder but prior to the die area. The metering pump has pressure transducers set to maintain the pressure into the pump and into the die area at constant predetermined pressure. By controlling the speed of the extruder screw sufficient material enters the metering pump and the die from the extruder. Laser micrometers downstream of the die are utilized to confirm that the cross-section is constant. The metering pump controls the speed of the extruder to maintain pre-set pressures at the pump input. Pressures are maintained by pressure transducers which send electrical signals to the controller unit of the pump which in turn controls the speed of the extruder. The laser micrometers may also be placed in a feedback loop to control the constancy of the rod or plate cross-section.